1. Field of the Invention
The field of the invention relates to the alignment of radiation instruments such as therapeutic machines and simulators used for cancer treatment.
2. Brief Description of the Prior Art
Various types of radiation equipment require that alignment be checked prior to actual use to insure the radiation will be directed precisely to the target and not elsewhere. Therapeutic machines such as linear accelerators and cobalt treatment machines are two types of such equipment.
Radiation therapy machines used in the radiation oncology departments of hospitals generally include a radiation head mounted to a rotatable gantry. Radiation should be directed by the head to the same point, the isocenter, regardless of the rotational position of the gantry or collimator.
In addition to including means for generating ionizing radiation, the radiation head generally includes an ordinary light source for generating a non-ionizing light beam upon the patient prior to therapy. The head may further include means for generating a target shadow also known as a "cross hair", which becomes visible upon the patient when the light source is actuated. The cross hair is used as one step in insuring that the radiation, such as x-rays, gamma rays or electrons, is directed to a properly positioned patient.
Since the physician or technician must assume that the ordinary light beam is directed at the same point as the subsequently applied therapeutic radiation, it is important that this, in fact, be the case. The conventional method of establishing light/radiation coincidence is to use x-ray film. The film, in its envelope, is first punctured with a needle on the borderline of the light field. It is then subsequently exposed to the radiation. The degree of overlap between the hole marks on the film and the radiation edge indicate the coincidence between these two fields. This technique has several significant drawbacks, namely the subjective marking of the light field and the length of time necessary to process the film. Scanning equipment is also available for scanning the light and radiation fields and determining the coincidence between the two fields and their widths.
A plurality of lasers are also conventionally used to properly position a patient. The lasers are oriented such that each of the beams eminating therefrom intersects each other at the machine isocenter. The beams impinge upon markings upon a patient to insure the patient is positioned to receive radiation from the radiation head only in a specific area. U.S. Pat. Nos. 4,123,660 and 4,223,227 disclose instruments for aligning lasers which include mirrors for detecting any divergences from the main beams.